1. Field of the Invention
The present invention relates to electric arc generation circuits. More specifically, the present invention relates to circuits in which such arcs are generated from an A.C. voltage.
2. Discussion of the Related Art
FIG. 1 illustrates a conventional example of a circuit 1 generating electric arcs adapted to igniting a fuel gas, for example, in an industrial enclosure, or in a domestic application to a gas oven, to the burners of a gas cooker, or to an independent gas lighter. Circuit 1 includes two input terminals I1 and I2 for receiving an A.C. supply voltage Vac, for example, the mains voltage, typically 220 volts at 50 Hz (or 110 volts at 60 Hz). Circuit 1 includes, in series between its terminals I1 and I2, a resistor R1, a diode DR, a capacitor C, and a primary winding L1 of an isolation transformer including one or several secondary windings L2. A pair of electrodes (not shown) is associated with each of secondary windings L2 of the transformer. Each pair is at a location where an electric arc is required, and its electrodes are at a small distance from each other. Circuit 1 also includes, in parallel with capacitor C and winding L1, a means FLC for organizing the charge and discharge of capacitor C. Means FLC includes, in antiparallel, a cathode-gate thyristor Th1 and a diode D1. The gate of thyristor Th1 is connected to the anode of a zener diode DZ1, the cathode of which is connected to the anode of thyristor Th1.
The operation of circuit 1 is described hereafter in relation with FIGS. 2 and 3. FIG. 2 illustrates the shape of current Ic in capacitor C (FIG. 1). FIG. 3 schematically illustrates the shape of voltage Vc across the capacitor.
During positive halfwaves of voltage Vac, capacitor C charges via diode DR and resistor R1. More precisely, diode DR is on when voltage Vac is greater than charge level Vc and the capacitor charges. In all other cases and, in particular, during negative halfwaves of voltage Vac, diode DR is blocked (nonconducting). All along the charge of capacitor C, thyristor Th1 is off.
It should be noted that several positive halfwaves of voltage Vac are necessary to reach the required charge level, controlled as described hereafter.
Indeed, as illustrated in FIG. 3, as long as voltage Vac has not reached a level VZ1, the capacitor pursues its charge at each halfwave. This results, for the shape of voltage Vc, in an increase by steps corresponding to several halfwaves and, for current Ic, to an exponential decrease of the maximum amplitudes of the charge current peaks along the halfwaves.
When voltage Vc reaches threshold VZ1, for example, 250 volts, determined by zener diode DZ1, the latter starts an avalanche (time t1) and a gate current triggers transistor Th1. Diode DR blocks and the capacitor abruptly discharges into primary winding L1 via the thyristor which is then used as a free wheel component. This discharge creates in the primary winding an increase (negative peak P with the sign conventions of FIG. 2) of the current which is reproduced at the secondary (by conservation of the magnetic energy). On the secondary side, this current variation results in an overvoltage across each winding L2 and thus across the corresponding electrodes, which creates an electric arc across these electrodes.
Thyristor Th1 turns off as the current flowing therethrough disappears, that is, when the capacitor is completely discharged. Diode D1 is then used as a free wheel diode to discharge the current associated with the reactive energy of the transformer as long as diode DR has not turned back on.
Then, at the beginning of the positive halfwave following this discharge (this arc), diode DR turns back on and capacitor C starts charging again in the way previously described until a time t1' when voltage Vc reaches level VZ1 again.
The time interval (t1'-t1) between two discharges (arcs) corresponds to the capacitor charge time before reaching threshold VZ1. This time thus conventionally depends on the level of supply voltage Vac.
This is a disadvantage of this type of electric arc generation circuits, since supply voltage Vac, for example, the mains voltage, can have random level variations. Such variations cause variations of the electric arc emission frequency.
If the level of voltage Vac increases, the capacitor charges faster, the arc frequency increases and can become too high, typically almost 10 Hz, and problems of electromagnetic compatibility arise.
This frequency problem also occurs in circuits where for other reasons, the arcs are synchronized on the frequency of voltage Vac. A drawback is then that the arc frequency is equal to the frequency (50 Hz) of voltage Vac, which is too high. This is the case of the gas lighter disclosed in UK application No. 2 130 646.
If the level of voltage Vac decreases, the capacitor charges slower, the arc frequency decreases and can become too low, typically under 2 Hz, which causes an accumulation of the gas to be ignited. Such an accumulation can raise security problems.
Now, electric distribution companies do not guarantee a constant voltage. The voltage can vary, for example, by .+-.20% with respect to the 220-volt mains voltage at 50 Hz. A value of 170 volts then corresponds to a 1.6-Hz arc frequency, and a value of 250 volts corresponds to a 6-Hz arc frequency.
Another disadvantage of a conventional circuit such as shown in FIG. 1 is that threshold VZ1 of zener diode DZ1 is likely to have variations due to technological dispersion and to operating drifts (temperature, etc.). Now, this threshold determines the arc periodicity.